Auditory profile analysis of harmonic signals

Spectral shape discrimination for harmonic complexes with 100-, 200-, or 400-Hz fundamental was investigated in two sets of experiments. In the first set, the signal was an increment in a single component of an otherwise equal-amplitude complex. The results of these experiments showed nearly a 30-dB increase in thresholds as the signal frequencies increased from 1000 to 5000 Hz. A transformation of data based on the assumption that the critical detection quantity is the change in the level in a critical band centered at the signal frequency was applied to remove the effects of the nonsignal components. The corrected thresholds have a bowl-like shape similar to that seen in studies of spectral shape discrimination of stimuli with components equally spaced on a logarithmic frequency scale. Additional experiments examined the effects of the number of components in the complex and the relative phase of the components of the harmonic complex. In the second set of experiments, the effects of local masking were examined either by increasing the level of several adjacent components, or by removing nonsignal components near a single signal component. Again, the results with harmonic signals are similar to those obtained with components spaced at equal intervals on a logarithmic frequency scale, if one calculates the increment in the level produced in a critical band centered at the signal frequency.

Auditory profile analysis: potential pitch cues

The ability to detect changes in spectral shape, or profile analysis, was measured for both complex and simple changes in the power spectrum of a complex equal-amplitude standard. In an effort to determine whether detectability was mediated by changes in pitch that are concomitant with changes in spectral shape, the pitch of the stimuli were altered on a trial by trial basis. For moderate-range pitch randomizations, thresholds were on average 3 dB poorer than when no pitch randomization was employed. For large-range pitch randomizations, threshold changes were larger, but performance levels remained above chance levels. The psychophysical data, coupled with computer simulations of Feth's envelope-weighted average instantaneous frequency pitch model, indicate that changes in pitch contribute little to the discriminability of complex spectra.

Frequency effects in profile analysis and detecting complex spectral changes

Seven experiments on the detectability of intensity changes in complex multitonal acoustic spectra are reported. Two general questions organize the experimental efforts. The first question is how the detectability of a change in a flat (equal energy) spectrum depends on the frequency region where a single intensive change is made. The answer is that frequency region plays a relatively minor role. Frequency changes in the midregion of the spectrum are the easiest to hear, but thresholds increase by only about 5 dB over the range from 200 to 5000 Hz. For all frequencies, the psychometric function is of the form d' = k(delta p), where k is a constant and delta p is the change in pressure. The second question is how can we predict the detectability of complex changes over the entire frequency range from the detectability of change at each separate region. Thresholds for detecting a change from a flat spectrum to a spectrum whose amplitude varies in sinusoidal ("rippled") fashion over logarithmic frequency are measured at different frequencies of ripple. The thresholds are found to be independent of ripple frequency and are 7 dB higher than predicted on the basis of an optimum combination rule.